Bias-selected n-p.sup.+ -n mercury-cadmium-telluride (HgCdTe) detectors that are sensitive to infrared (IR) radiation within two spectral bands or "colors" have been fabricated using various combinations of molecular beam epitaxy (MBE) and liquid phase epitaxy (LPE). These detectors include those fabricated solely by MBE, those fabricated by the MBE growth of a long wavelength IR (LWIR) layer on top of an LPE grown mid-wavelength IR (MWIR) p.sup.+ -n double layer, and those fabricated using only LPE.
However, a structure grown using MBE typically exhibits a high leakage current and low R.sub.o A. This is due to the immaturity of the MBE growth process relative to LPE.
Commonly assigned U.S. Pat. No. 5,113,076, "Two Terminal Multi-band Infrared Radiation Detector" to E. F. Schulte, discloses a radiation detector having two heterojunctions that function in a manner analogous to two back-to-back photodiodes. Each of the photodiodes is responsive to radiation within a different IR spectral band, such as LWIR and MWIR. Disclosed configurations include an n-p-n configuration, a p-n-p configuration, and a p-n-p-n configuration.
Reference in this regard is also made to a n-p+-n dual-band detector described by J. M. Arias et al. in the Journal of Applied Physics, 70(8), Oct. 15, 1991, pg. 4820-4822.
Commonly assigned U.S. Pat. No. 5,149,956, "Two-Color Radiation Detector Array and Methods of Fabricating Same", by P. R. Norton, teaches an array of three layer n-p-n or p-n-p photodetector structures in which a middle, electrically common layer is electrically and physically continuous across the array. The top and bottom layers are sensitive to different spectral bands, and each unit cell has two topside electrical contact structures, including indium bumps, for interfacing these layers to a readout circuit. A common contact provides a connection to the middle layer. A binary or analog lens array can be provided on or adjacent to the back, radiation receiving surface of the detector array to improve the fill factor. Due to the geometry of this device, the indium bumps are positioned at different levels.
In commonly assigned U.S. patent application Ser. No. 08/045,741, filed Apr. 8, 1993, "Dual-Band Infrared Radiation Detector Optimized for Fabrication In Compositionally Graded HgCdTe", now U.S. Pat. No. 5,457,331, K. Kosai and G. R. Chapman describe a four layer n-p.sup.+ -p-n.sup.+ structure, grown by the well-characterized LPE process, that includes a substrate and, from a bottom layer next to the substrate to the surface: (a) a MWIR radiation responsive low-doped n-type absorbing layer; (b) a high-doped p-type (p.sup.+) cap layer; (c) a LWIR radiation responsive low-doped p-type absorbing layer; and (d) a high-doped n-type (n.sup.+) top layer. The high-doped n-type top layer has a compositional profile that is similar to the high-doped p-type cap layer.
For an application where it is desirable to obtain the simultaneous detection of radiation in two spectral bands, and to also employ a topside readout integrated circuit, electrical contact must be made separately to the MWIR radiation responsive n-type layer, the LWIR radiation responsive p-type layer, and to the n.sup.+ top (capping) layer. Alternatively, the structure may be fabricated as a n-p+-n structure and electrical contact made to all three layers. The electrical contact is preferably made via indium bump interconnections so that the detector unit cell can be subsequently hybridized with the electronic readout integrated circuit.
For this case a problem is presented in that the three indium bump interconnects must be provided within the limited area (for example, 50 microns square) of the unit cell. Furthermore, it is desirable to electrically isolate the individual unit cells of the array from one another. Satisfying both of these requirements can result in a decrease in the active optical area, with a corresponding reduction in sensitivity. A further desirable goal is to provide the top-most point of each indium bump at approximately the same height, relative to the underlying unit cell, so as to facilitate the interconnection of the unit cell to a readout integrated circuit.